gnttab: introduce maptrack lock

Split grant table lock into two separate locks. One to protect
maptrack free list (maptrack_lock) and one for everything else (lock).

Based on a patch originally by Matt Wilson <msw@amazon.com>.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

docs/misc/grant-tables.txt

@@ -87,6 +87,7 @@ is complete.
                                inconsistent grant table state such as current
                                version, partially initialized active table pages,
                                etc.
+  grant_table->maptrack_lock : spinlock used to protect the maptrack free list
   active_grant_entry->lock   : spinlock used to serialize modifications to
                                active entries
 
@@ -94,6 +95,9 @@ is complete.
  that access members of struct grant_table must acquire the lock
  around critical sections.
 
+ The maptrack free list is protected by its own spinlock. The maptrack
+ lock may be locked while holding the grant table lock.
+
  Active entries are obtained by calling active_entry_acquire(gt, ref).
  This function returns a pointer to the active entry after locking its
  spinlock. The caller must hold the grant table lock for the gt in

xen/common/grant_table.c

@@ -288,10 +288,10 @@ static inline void
 put_maptrack_handle(
     struct grant_table *t, int handle)
 {
-    spin_lock(&t->lock);
+    spin_lock(&t->maptrack_lock);
     maptrack_entry(t, handle).ref = t->maptrack_head;
     t->maptrack_head = handle;
-    spin_unlock(&t->lock);
+    spin_unlock(&t->maptrack_lock);
 }
 
 static inline int
@@ -303,7 +303,7 @@ get_maptrack_handle(
     struct grant_mapping *new_mt;
     unsigned int          new_mt_limit, nr_frames;
 
-    spin_lock(&lgt->lock);
+    spin_lock(&lgt->maptrack_lock);
 
     while ( unlikely((handle = __get_maptrack_handle(lgt)) == -1) )
     {
@@ -332,7 +332,7 @@ get_maptrack_handle(
                  nr_frames + 1);
     }
 
-    spin_unlock(&lgt->lock);
+    spin_unlock(&lgt->maptrack_lock);
 
     return handle;
 }
@@ -3017,6 +3017,7 @@ grant_table_create(
 
     /* Simple stuff. */
     spin_lock_init(&t->lock);
+    spin_lock_init(&t->maptrack_lock);
     t->nr_grant_frames = INITIAL_NR_GRANT_FRAMES;
 
     /* Active grant table. */

xen/include/xen/grant_table.h

@@ -82,6 +82,8 @@ struct grant_table {
     struct grant_mapping **maptrack;
     unsigned int          maptrack_head;
     unsigned int          maptrack_limit;
+    /* Lock protecting the maptrack page list, head, and limit */
+    spinlock_t            maptrack_lock;
     /* Lock protecting updates to active and shared grant tables. */
     spinlock_t            lock;
     /* The defined versions are 1 and 2.  Set to 0 if we don't know